Download Molecular Self-Assembly and Nanomanipulation

ESSAY
Molecular Self-Assembly and
Nanomanipulation ± Two Key
Technologies in Nanoscience and
Templating**
By Wolfgang M. Heckl*
If at some time in the future we will want to build objects
out of single atoms and molecules so that we can manufacture them with molecule assembly machines then we must
think about which techniques need to be mastered in order to
move things on the scale of atoms or molecules.
The first task will be to position the atoms and molecules,
and the second will be to know if they have been correctly positioned. The scanning probe microscope has given us a wonderful method of seeing and working down to the single
atom level. The tip of the microscope is so designed that in
some cases it can also be used for positioning.
When searching for a parallel assembly technique for
atoms and molecules one is well advised to look at the methods of nature. Nature uses the method of self-assembly. Several examples are shown below to illustrate that self-assembling systems can be used as templates, to structure, order,
and position given entities as may be required in futuristic
applications. That one cannot always predict precisely the
outcome of a self-assembly process is first illustrated by a
short story.
This story took place in the 1950's, perhaps in an English
garden. It was a beautiful romantic summer evening, the
moon had risen, a young lady sat on a park bench and an old
man came along the path. The young lady looked at the old
man and said: ªImagine Albert, if we had a child together. It
would have your brain and my beauty.º The old man was
wise enough to answer, ªbut Marilyn, just imagine how it
would be if the opposite happenedº. He knew about the stochastic nature of self-organisation and it is no different when
we talk about the passing on of hereditary information. It is
then dependent upon whether we are speaking of a deterministic self assembly process and if the individual units will be
dominant or recessive. On the nanoscale we often cannot be
precise, even though we know that molecules interact with
each other across weak hydrogen bonds in the A-T-G-C- pattern. We do not know what results: whether it is a deterministic process or if it is a process which is unpredictable, with
unexpected consequences.
ADVANCED ENGINEERING MATERIALS 2004, 6, No. 10
There is a vision in the distant future that one day it will
be possible to create whatever we dream of, like a deus ex
machina, using molecular components and atoms. We would
like to have a machine: ªthe vision is that small active units
assemble themselves together. They do it in countless nanofactories without human aid. By them every desired material,
every substance required would be put together atom by
atom, molecule by molecule. (taken from Wolfgang M. Heckl
in ¹Das Nanoschnitzel, Vision und Wirklichkeit in der Nanotechnologie,ª Bayerischer Rundfunk, Fernsehen 2003 ¹The
nanosteak, the vision and reality of nanotechnologyª Bavarian TV 2003). The vision goes a few steps further. Here is an
example from the area of medicine, from the science fiction
novel ªPreyº by Michael Crichton. It imagines that self assembled nanoparticles can be used as a camera: ªI have in
this syringe nanocameras only two billionths of a millimetre
long. Compare the size with a red blood cell. You can see that
our cameras look like tiny octopuses. I will now inject millions of these cameras into the blood stream of our patient.
The cameras will form a swarm and immediately show a picture of the vein. They can even show a single blood cell. This
method makes it possible to see into the smallest blood vessels, even the capillaries of the finger tips.º
The complexity of nature arises from self organisation. A
simple example is a school of fish. Each individual does not
think for itself, it is governed only by group dynamics: keep a
constant distance from the neighbouring fish, do only what
the neighbour fish does. The result is that the shoal behaves
like a single complex individual which moves itself in a way
±
[*] M. Heckl
LMU München
Theresienstr. 41, 80333 München, Germany
E-mail: [email protected]
[**] This text is based on a talk given at the Nanomat-Szene Meeting in Karlsruhe, Germany, on April, 01, 2004.
DOI: 10.1002/adem.200404493
2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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ESSAY
Heckl/Two Key Technologies in Nanoscience and Templating
in which none of the individuals could have intended. This is
peculiar to emergent systems which self-assemble. Such a
primitive form of self assembly is much more than crystal
growth. In his novel ªPreyº Michael Crichton plays with such
a shoal of hybrids: living beings in this case viruses which hybridise with inorganic self assembling machines.
These machines present a danger which is currently being
strongly publicly discussed. The danger belongs in the realms
of science fiction. It must however be discussed scientifically
because it is being discussed publicly. Scientists no longer
have the position of researchers who can say that they have
everything under control and won't argue against public concerns.
There will be however the opportunity to use nanomachines for something positive, e.g. the removal of toxic substances from a waste disposal site, to make new non-toxic
materials from the waste atoms and molecules, even to build
useful substances. The nanomachines could be first frozen
and stored perhaps in ice cubes or polymer matrix and transported. At a certain temperature they could be set free to do
their work. A fantastic scenario.
Self-assembling nanomachines are not unusual. One example is the muscel system comprised of actine and myosin
proteins. One of these proteins acts as a machine and makes
nano meter long steps along the protein strand, this occurs
each time a muscle is moved. Another example is given by
the ribosome present in each cell, which is actually a nano-assembling machine which reads the DNA and translates the
code into protein. It works wonderfully in nature. The difficulty is to mimic the idea and to use it in practicable technology. This type of Nanobionic requires a second type of evolution. This evolution II is the hole idea of Nano.
Jean-Marie Lehn defined Self-organisation as a spontaneous emergence of supramolecular structures arising from
individual components, such as atoms and molecules, under
certain conditions. The proteins, such amazingly complex
structures, are a very good example of this, as also are single
macromolecules or polymers.
How does self-assembly on the nanoscale work? Taking
DNA as an example: In experiments we have observed that
DNA bases, the letters of the genetic code, self-organise into a
two dimensional molecular crystal when applied in liquid solution onto a mineral surface. This amazing spontaneous formation of order, the arising of something new from the single
molecular components occurs in a lsecond or less with 1016
molecules per square centimetre. This is a phenomenon
which could never be imitated if it had to be built molecule
by molecule according to a fixed plan. This can only happen
by a natural process of self-assembly. If DNA or RNA bases
are allowed to self-organise on a surface it is always the case
that hydrogen bonds form between the molecules and hold
the structure stable. Self-assembly of DNA molecules is one
of the most important themes for research because it leads to
an explanation for the origin of life, one of the greatest research challenges of our century. In fact, the origin of life was
nanotechnology 4.8 billion years ago.
Table 1. Self-assembling molecules to form two-dimensional crystals on surfaces.
Molecule
Sum formula
rel. Molecular mass
Subl. -Enthalpy
[kJ/mol]
measured Subl.-Temp.
[Celsius]
Adenine (A)
C5N5H5
135,1
126,3
205
Guanine (G)
C5N5OH5
151,1
186,2
270
Cytosine (C)
C4N3OH5
111,1
167
220
Thymine (T)
C5N2O2H6
126,1
131,3
153
Uracil (U)
C4N2O2H4
112,1
126,5
168
C4N2O S H4
128,2
129
155
C4N O2H5
75,1
136,5
130
C24O6H8
392,3
310
Coronen
C24H12
300,35
100
Trimesic Acid
C9O6H6
210,1
181
Indigotine
C16H10 N2O2
262
in 8-CB solvent
Quinacridone
C20H12 N2O2
320
in 8-CB solvent
C14H8O4
240
in 8-CB solvent
2-Thiouracil (2-TU)
Glycine (Gl)
PTCDA(Perylene-tetra-carbonic acid)
Alizarine
Alizarine Krapplack
Hemine (Protoporphyrine)
in 8-CB solvent
C32H28 ClFeN4O4
616
6-, 8,- 10-,12- Cyanobiphenyle
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in 8-CB solvent
From solution
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Heckl/Two Key Technologies in Nanoscience and Templating
An essential factor in the development of life forms is chirality, without it life does not exist. We could experimentally
recognis this very early on with the help of a scanning tunnel
microscope, DNA bases grow either right or left handed on
mineral template surfaces. The importance is that a chiral
break in symmetry must occur for single nucleic and amino
acids to give rise to life in nature. We have investigated a
whole series of molecules which self-assemble to form such
two dimensional crystals on surfaces. They have molecular
weights of up to 616 atomic units e.g. hemine, a protoporphyrin molecule (see Table 1).
Several more recent examples of molecular self organisation from our current work are described below: Trimesic
acid, a benzene ring with three additional carboxyl groups,
the dye molecule Coronene and the semi-conducting quinacridone molecule (see Fig. 1).
The first molecules investigated with the scanning tunnel
microscope were liquid crystal molecules. The phases separate, the liquid crystal takes different structures and there is a
phase boundary between them. That is actually the first molecule which was directly seen by the real space method back
in 1988 by Foster and Frommer. Other structures can be
formed from liquid crystals. For example one is of particular
interest because it is open-pored and therefore is a two-dimensional guest system in which one can store hosts.
Carmine is a red substance known to every artist. It crystallises spontaneously in two-dimensions by solid-solid wetting on a surface. There are of imperfections in the structure,
however the self-assembly process works very well.
Quinacrodon is another pigment with a linear type structure. Each molecule has two hydrogen bonds bridging the
neighbouring molecules. A one-dimensional chain can be
made. Tunnelling spectroscopy reveils that these chains are
semi-conducting, and so can be used as molecular wires. The
wires can be moved and repositioned with the tip of the microscope to assists the self-assembly process. Specific structures can be made such as a condensor geometry or a structure where three conducting structures come together in a
transistor geometry. It is however a huge challenge to connect
these structures with the outside world; that is to electrodes.
Two layers can be placed on top of each other to form a
crossed pattern.
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C60
TMA template
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mechanical - kick
Room Temperature STM Manipulation of Single C60 Molecules at the Liquid Solid
Interface - Playing Nano-Soccer, S. J.H. Griessl, M. Lackinger, F. Jamitzky, T. Markert,
M.Hietschold and W. M. Heckl, J. Phys.Chem, submitted
Fig. 2. Room temperature STM manipulation of single C60 molecules at the liquid solid interface ± playing nano-soccer.
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Fig. 1. 8-Cyanobiphenyl-cavities for an organic host-guest system
If the conductors grow in different directions a technique
we have termed ªMolecular Combingª can be used to align
the molecular wires.
The most successful Host-Guest system is a two dimensional lattice prepared in our laboratories using trimesic acid.
In the literature a three dimensional crystal of trimesic acid
has been described. In 1998 our molecular mechanics simulation illustrated how this molecule could form a two dimensional layer on a solid surface. From this we could predict the
formation of a trimesic acid lattice with an open-pored template structure. The substrate we first used was silicon and
this led to some very disappointing results because the adsorption energy is too high and so the expected open-pored,
self-organised, crystalline two-dimensional lattice did not
arise. We concluded therefore that the substrate needs to have
a much lower adsorption energy, a possible suitable substrate
was found to be graphite. Using graphite an open-pored
structure was formed where six molecules held together by
stereospecific hydrogen bonds form a cavity with a distance
between the cavities of around 2 nm. We have termed the
structure a ªchicken-wireº structure since it resembles the
macroscopic fence. We have also succeeded in making a
ªFlowerº structure where smaller cavities are formed next to
the larger ones. In this structure six trimesic molecules form a
ring with six more molecules hexagonally surrounding them.
Here additional hydrogen bonds form in trigonal geometry
and not only in the geometry with 180 rotation.
The open cavities, i.e. the ªguestº positions, can be filled
with guest molecules. An interesting experiment can be done
here. By application of the tip of the scanning tunnel microscope to a trimesic molecule in a guest position the molecule
can be switched from the corresponding top position to a
down position. In addition to this vertical transposition we
have also resolved a horizontal switching process. There are
six docking positions for a trimesic molecule in such a ca
2 nm sized cage. The six positions can be actively rotated
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Heckl/Two Key Technologies in Nanoscience and Templating
1nm
Fig. 3. The structure of the trimesic network can be regarded as a two-dimensional zeolite.
Fig. 4. Coronene molecules (blue), ªkickedº out by a nanomanipulation technique.
these early stages in the evolution of life by using primeval
from the outside. This corresponds to an elementarymolecusoup scenarios on mineral surfaces, for example how nucleic
lar switch.
and amino acids can self-assemble to form layers on minerals
Other molecules which are very interesting are carbon
such as pyrite and so represent the first step towards life.
cage molecules, e.g. the C60- molecule, also known as BuckDarwin knew that the key was the formation of a protein or
minster fullerene. They have exactly the right size to fit perpolypeptide de novo. An abiotic situation needs to be imafectly in the cavities of our molecular template trimesic netgined with no bioreactors present that could have manufacwork. One of these football shaped molecules has been
tured these molecules. Our method is to drop a solution of
placed in the middle of one of the template rings, and can be
nucleic and amino acids on a mineral surface. This imitates
ªkickedº into the guest position of the neighbouring ring.
the scenario where a lake of primeval soup evaporated on hot
This shows that manipulation on the molecular scale is possistones. We have called the method the Sizzling technique, beble (Fig. 2).
cause of the noise made by the water lapping on the hot
Another molecule of particular interest is coronene. Quanrocks. A completely spontaneous two-dimensional ordering
tum mechanical effects can be directly observed, the Fresnellof organic molecules occurs under the simplest conditions in
oscillation and the scattering of surface electron waves on the
a fraction of a second. The mineral surface acts as a template
single molecules even at room temperature. Interference effor the self-assembly of the nucleic acids which in turn acts as
fects can even be observed between two closely neighbouring
an organic template for amino acids. This is the coding mechmolecules (Fig. 3).
anism: nucleic acids form the code for amino acids which
In Figure 4 coronene molecules (blue) have been ªkickedª
subsequently hydrogen bond on top of them, able to polyout by our nanomanipulation technique, an elementary reacmerize now into enzymes and other proteins (Fig. 5).
tion. There is however space around the molecule within its
cavity, and it is therefore at least able
to vibrate (green are bound coronen
molecules and blue are vibrating molecules). This has been reported in the
literature. Gimzewski has reported a
case of molecules either remaining
still or turning depending upon
where they are attached. This was
described as a molecular motor. We
believe that these and our molecules
quite simply vibrate. The term ªrotateº is inconsistent with the second
law of thermodynamics.
Returning now to the original
theme. I believe that the emergence of
living systems has been a result of
self-assembly on the nanoscale, essentially the self-assembly of nucleic
and amino acids. We have been
Fig. 5. Spontaneous two-dimensional ordering of organic molecules on a mineral surface, acting as a template.
working for many years investigating
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amino acids and peptide like systems are formed. The formation of the peptide bond is fundamental to the formation of
enzymes from amino acids. This is a de novo polypeptide
synthesis. A cycle of coded enzyme and amplifications arises
by itself. We hope therefore to have shown experimentally
nothing less than the origin of the molecular assembly of life,
the principles of its spontaneous emergence from the simplest
natural components.
______________________
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ESSAY
In cells there is a three dimensional process. The DNA is
read in the ribosomes and for each triplet of nucleic acids an
amino acid is built into the forming protein in this ªmolecule
assemblerº. Nucleic acids code for amino acids by direct
stereospecific interactions. In our experiments there are no
ribosomes, but there were also none at the beginning of life.
There were molecules floating in the primeval lake and we
imitate the phenomenon of self-assembly in our ªhistoricº experiments. We cover a surface with nucleic acids, then with